The present invention relates to a motor-driven fuel pump, and more particularly to a compact motor-driven fuel pump producing a large output for use in a fuel supply system for an automobile.
There has been provided a motor-driven fuel pump of a type in which a pumping unit, a brushless electric motor for driving the pumping unit and a control circuit for controlling power supply to the brushless electric motor are encased within an integral pump casing, as described in, for example, Japanese Patent Laid-Open Publication Nos. 56-88982 and 61-14496.
In such a prior art motor-driven fuel pump, the control circuit only includes a circuit for sequentially switching stator coils through which current flows in accordance with the angular position of a rotor of the brushless motor. This causes current flow as shown in FIG. 4 on starting of the motor-driven fuel pump. Specifically, immediately after starting of the pump, current begins to flow through the stator coil, the current progressively increasing, for example, as shown by a waveform A as a parameter of inductance component, resistance component and the like of the stator coil. Then, as the torque generated by the motor comes near the load torque, the current becomes stable, until it reaches an amperage determined by the output characteristic of the motor and the load torque, and is maintained to flow at the amperage thereafter. Immediately after starting, because of low rotating speed of the rotor, the current flowing through the stator coil increases to a saturation amperage (the saturation amperage in FIG. 4 is 30 to 35 amperes), and thereafter the current is maintained at the saturation amperage. As the rotating speed of the rotor is gradually increased until it reaches a normal operating speed, the current flowing through the stator coil becomes drastically smaller than the saturation current, for example, to an amperage of about 10 amperes, as shown by waveforms in the region B.
Therefore, the prior art motor-driven fuel pump must employ elements for switching the stator coil current, such as, power transistors which can switch a large current (of 30 to 35 amperes in the above description) flowing immediately after starting.
In a cold district, jumper start means may be employed at starting of an engine. This means is adapted to apply voltage twice the normal operating power voltage (for example, by connecting two batteries in series), at the starting of the engine. Such jumper start means causes a current greatly larger than the normal operating current to flow at the starting of the motor-driven fuel pump.
Thus, the prior art motor-driven fuel pump is at a disadvantage because requires elements for switching a large current which is quite a high amperage in comparison with the normal operating current of about 10 amperes and flows only immediately after starting.